Analog RF (Radio Frequency) and microwave Fiber-Optic Links are commonly used in the transmission of signals from antennas and electrical sensors from remote locations.
The analog RF electrical signal is first converted into optical signal using an electro-optic modulator. Then the optical signal is transmitted through the optical fiber to another location where the optical receiver converts the optical signal back to an electrical signal.
There are many ways to implement an analog fiber-optic link. One of the most basic analog fiber-optic link employs a laser source, an electro-optic modulator for converting the electrical signal into optical signal, optical fibers as the transmission medium and a photo-detector for converting the optical signal back to an electrical signal. The electrical signal, with frequencies ranging from DC to >100 GHz, can be converted to optical signal using an electro-optic modulator.
Modulated analog fiber-optic links are widely used. The most common analog RF fiber-optic link uses an electro-optic intensity modulator to convert the electrical signal into an optical signal. There are several types of intensity modulators. The most commonly-employed intensity modulators for that application are Mach-Zehnder (MZ) Interferometric modulators, based on lithium niobate (LiNbO3) electro-optic waveguide technology. There is ample literature on this type of analog RF fiber optic links using MZ modulators.
An MZ interferometric intensity modulator is a simple device in which the optical transmission characteristic, as a function of the applied input voltage to the device, is in form of a sinusoidal function. In general, the MZ modulator needs to be biased with a DC voltage to set the operating point at half-power transmission point of the sinusoidal transfer function. This half-power operating point is where the optical transmission vs. applied voltage is at maximum linearity, and the second order derivative is zero. This MZ intensity modulation fiber-optic link, with MZ biased at this half-power point, is commonly used for wideband (multi-octave) RF analog signal transmission.
However, the performance of an MZ-based fiber-optic link is limited by issues associated with the operating point/DC bias voltage stability, which can be affected by many factors including environmental conditions such as changes in temperature. If the operating point is not exactly at the half-power point, the linear transfer characteristic is affected. This will result in degradation in the performance of the fiber-optic link, particularly the spurious-free dynamic-range, due to intermodulation signal distortion (2nd, 3rd order and etc.). There is ample literature on this subject.
Since the electro-optic modulator needs to be right at the RF sensor or antenna at the remote site, the modulator is typically subjected to greater temperature variation and other environment factors. A DC bias voltage is required to be applied to the modulator, and the proper bias voltage also needs to be adjusted and tracked so that the operating point of the MZ modulator remains at the maximum linearity point. The need for the DC bias voltage and tracking electronics means electrical power is needed at the remote site. This requirement is undesirable for many applications, and tracking electronics can also adversely affect the performance of the overall fiber-optic link.
In addition, even with the operating point maintained at the half-power, 2nd derivative null point, the spurious-free dynamic range of a MZ intensity modulated link is still limited by the 3rd order intermodulation distortion caused by the limited linearity of the sinusoidal transfer function of the MZ modulator. This is typically the main limiting factor in the spurious-free dynamic-range of a MZ-based analog fiber-optic link.
In order to achieve higher dynamic-range, another type of modulator with enhanced linearity is required. However, these “enhanced linearity” modulators are very difficult to achieve in practice without some performance trade-offs. In addition, an enhanced linearity modulator often means more complex operating point control and feedback electronics. To apply such a device in a remote location has proved to be difficult and impractical.
Phase-modulated analog fiber-optic links can be used. Instead of using a MZ intensity modulator, a phase modulator and a simple optical delay-line filter can be used to construct a bias-free phase-modulated analog fiber optic link. The approach is described in literature.
The benefit of such implementation is that the electro-optic phase modulator does not require any DC bias voltage and, thus, can be placed at the antenna/RF sensor site without the need for any control bias electronics. The optical delay-line filter can be placed far away from the phase modulator via the optical fiber transmission line at the receiver site. In general, the receiver site can be located anywhere and therefore can be located in a controlled environment, in contrast to the antenna which is typically in an open environment exposed to various elements.
It has been shown that the phase modulated link using a phase modulator and a simple delay-line filter can achieve performance similar to that of the MZ link, using suitable design parameters as described in literature. Unfortunately, the spurious-free dynamic-range of this bias-free phase modulated link using a single delay-line optical filter also is limited by the 3rd order intermodulation, similar to that of the MZ link.
Problems which need solutions continue to exist. This limitation on the spurious-free dynamic range of a typical Analog Fiber-Optic link due to this 3rd order intermodulation distortion makes the link, based on either MZ intensity modulation or Phase modulated link with delay-line filter, not adequate for many higher performance systems, in which much higher spurious-free dynamic range is required.
There is a critical need to develop a higher performance Analog RF Fiber-optic link that can achieve a much greater dynamic-range, yet is simple to implement and operate.